A North Korean defector working in Seoul City Hall has been arrested on suspicion of spying, after allegedly handing over a list of 10,000 defectors living in Seoul to the North Korean government. The incident is not just a gross violation of South Korea’s national security and the personal security of those whose personal details were on the list. It also represents a major setback in the already challenging process of building trust between North and South Koreans as part of integrating defectors into society.

The man, named only as Mr. Yoo by South Korea’s National Intelligence Service, came to